an equal number of cups of sugar, eg. for 5 cups of juice, use 5 cups of sugar

one gallon (or larger) kettle
quarter or half pint, or pint canning jars
lids and rings
spoons, canning funnel, small dishes, etc
paper towel
It's best if some of the crabapples that you used to make the juice weren't fully ripe, as they will provide more pectin, but apples have enough natural pectin that crabapple juice usually gels well without adding any commercial product.
However many cups of juice you begin with, you'll end up with slightly less jelly. So if you want four full half pints of jelly, start with a bit more than 4 cups of juice. I began with 4 cups of juice, and the picture looks as if I got four jars of jelly, but one was not full enough to seal. Not a problem- we ate it up over the next few days! But if you want to produce a certain number of jars for gifts, or something, you don't want to be short.
Pour the measured juice and sugar into a kettle. You don't want the kettle to be very full or you'll have an awful spatter mess on the stove. A large surface area, and shallow depth of liquid, is better than a narrow, deep pan.
Bring to a boil and continue to boil, without stirring, until the jelly stage is reached. This can take a while, possibly an hour or more. This is approximately 8 F degrees higher than the boiling point. (At most altitudes this will be 220 F, but you'll need to make adjustments for high altitudes.) Another good way to test without a thermometer is to check with a cold spoon. I keep a metal spoon in the freezer, and when I think the liquid is starting to thicken I use the cold spoon to dip a little bit from the pan. If it breaks off the spoon as a jelly, it's ready. I'll add a picture here when I make the next batch.
Turn off heat. Skim the foam with a metal spoon.

Ladle or pour into clean, sterile jelly jars, leaving 1/4 inch headspace. Having the jars sterile is really important so you don't have to process the jelly. Fill one jar at a time, so that the jelly is as hot as possible for each. Clean the rims, Place clean, softened lids on the jars, and add a ring. This is steps 4-8 of Basic Instructions for Hot Water Bath Canning.

Here's the way to sterilize the inside of the lid. When you have put the lid and ring on a jar turn the jar over and let it sit upside down for 30-60 seconds. Then turn it right side up again, and place the jars at least an inch apart. In this way, the hot jelly has come in contact with the lid. Because jelly has so much sugar, you can seal jars this way and not have them spoil. Eventually, I'll write a whole post about this step.

crabapple jelly on an English muffin (photo by jhy)

Let the jars rest without moving them until cool. The lids should seal without any problem. If one doesn't, you can re-heat the jelly and try again, or just move that jar to the fridge for consumption.
This is a lovely, tart jelly with a clear red color. It's good on bread products or to accompany meat.